Method for measuring stress/strain using Barkhausen noises

ABSTRACT

In a method of measuring stress/strain by detecting Barkhausen noise, an exciting/sensing device is arranged at least adjacent to a magnetic or magnetizable element, and passing an increasing magnetizing current through the exciting device. The start of the Barkhausen noise in the element, as a function of the magnetizing current is detected by the sensing device, and the magnetizing current at that time represents a measurement of the stress/strain condition of the element.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of German patent document 102 39017.7, filed Aug. 20, 2002 (PCT International Application No.PCT/EP2003/009085), the disclosure(s) of which is (are) expresslyincorporated by reference herein.

The present invention relates to a method of measuring stress/strain bymeans of Barkhausen noise, such as may be used, for example, to measurestress/strain in bolts or rivets at the wing or fuselage of an airplane,particularly during maintenance work.

For example, for tightening bolts, torque wrenches have been used, whilesimultaneously determining the internal stress in the bolt by means ofultrasonic measurements. However, this technique requires that the boltbe coated or laminated with a piezoelectric, which makes it considerablymore expensive.

In addition, it is known to use micromagnetic methods and sensors whichmeasure Barkhausen noise to detect material changes caused by treatmentprocesses, based on the proposition that the magnetic structure of thematerials is influenced by the characteristics of the material.Crystallite limits and other inhomogeneities (such as dislocations,foreign atoms and inclusions) hinder the movement of the so-called Blochwalls; and detachment of the walls from inhomogeneities results in“bounces” in the magnetization, or so-called “Barkhausen noise”. Thesejolt-type movements of the Bloch walls can be recorded in the form ofbrief electric pulses, using a coil. The number, height and intensity ofthe pulses depends on the material and its condition.

Such a method for the nondestructive evaluation of ferromagneticmaterials is known, for example, from German Patent Document DE 196 31311 C2. The measuring principle is based on the fact that, when aferromagnet is periodically reversed, the magnetic domain structure iscontinuously changed, and boundaries between areas of the samemagnetization (that is, Bloch walls) move through the material structureand interact with the microstructure of the material. This interactionis received as an electromagnetic signal—the so-called Barkhausen noise.

Such methods, which are based on an analysis of the Barkhausen noise,are generally used for quality control; for example, for optimizingdifferent treatment processes (grinding, heat treatment, etc.) ofcomponents. The components may, for example, be ground parts, camshafts,crankshafts, bearings, gear wheels, injection valves and numerous otherparts from automotive engineering and the aerospace field.

One object of the invention is to provide a fast and simple method ofmeasuring stress/strain.

Another object of the invention is to provide such a method which iscapable of determining stress/strain conditions in fastening devices.

These and other objects and advantages are achieved by the methodaccording to the invention, in which an exciting/sensing device isarranged at least adjacent to one magnetic or magnetizable element(preferably in a partial area around the magnetic or magnetizableelement), so that it is acted upon by a rising magnetizing current. Thesensing device thus detects the starting of the Barkhausen noise in theelement (which is a measurement of the stress/strain condition of theelement) as a function of the magnetizing current. Expediently, thestarting of the Barkhausen noise is determined by a comparativemeasurement relative to reference values.

The invention has the advantage that the physical effect of theBarkhausen noise is effectively utilized. The stress/strain condition ofa magnetic or magnetizable element (preferably ferromagnetic) isdetermined in a contactless manner by a simple mounting of anexciting/sensing device. In this case, the element is magnetized by themagnetic field generated by the exciting device, and the sensing devicedetects the Barkhausen noise.

In a preferred embodiment, the exciting/sensing device is constructed inone piece (preferably a single coil), which acts simultaneously as anexciting device and a sensing device. The sensing device detects themagnetizing current at which the Barkhausen noise occurs, themagnetizing current being proportional to the internal stress in theelement. Such an arrangement has the advantage that the equipmentexpenditures are kept as low as possible.

In an alternative embodiment, the exciting device is again constructedas a coil, but the sensing device for detecting the Barkhausen noise isan acoustic or interferometric detector, which is notable for its smalldimensions.

A pulsed magnetizing current is expediently used for excitation, andduring the off-time of the pulses, the sensing device is set to receivethe Barkhausen noise. This arrangement has the advantage that highermagnetic fields can be generated without thermally overloading theexciting device.

According to another preferred embodiment, an intermediate element of anon-magnetic or non-magnetizable material (e.g., a washer or the like)is arranged between the magnetic or magnetizable element (for example, afastening device in the form of a bolt) and a structure, such acomponent, which is to be connected therewith. This embodiment has theadvantage that it measures directly the stress existing in the fasteningdevice as a result of the fastening. The washer is optional andpreferably consists of a non-ferromagnetic material.

In an alternative embodiment, a magnetic or magnetizable element isfirst arranged between a non-magnetic or non-magnetizable fasteningdevice and a structure to be connected therewith. Advantageously,stress/strain conditions of a non-magnetic fastening device cantherefore also be determined by measuring the mechanical stress that istransmitted by the fastening device to a preferably ferro-magneticelement (for example, a washer).

The method according to the invention is preferably used when measuringstress/strain conditions of fastening devices (such as screwed orinserted bolts, rivets, etc.). The method can be used in multiplemanners, for example, in the maintenance of airplanes, helicopters,motor vehicles, etc.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a preferred embodiment forimplementing the method according to the invention;

FIG. 2 is a view of representations of experimental measured values ofthe Barkhausen noise;

FIG. 3 is a schematic representation of a measuring arrangement as analternative to FIG. 1; and

FIG. 4 is a schematic representation of an alternative measuringarrangement by means of a riveted connection.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a bolt 4 which is connected with a structure 6. As a rule,the bolt 4 is used for the fastening, fixing or holding of the structurewhich may comprise several components. The connection between the bolt 4and the structure 6 may be screwed, riveted, inserted or the like. Inthe case of the screwed bolt illustrated in FIG. 1, a washer 5 is as arule arranged between the head 4 a of the bolt and the structure 6.

For determining the stress/strain condition of the bolt 4, in a firststep, the coil, which has the reference number 1 in FIG. 1, is fittedover, onto or adjacent to the head 4 a of the bolt. According to apreferred embodiment, the coil is used both for generating a magneticfield and for detecting the Barkhausen noise, as described in greaterdetail hereinafter. However, instead of the single coil 1, separatecomponents can also be used: for example, a first coil for generatingthe magnetic field and a second coil which is used for detecting theBarkhausen noise. In the following discussion, reference will be made tothe alternative embodiment, in which the exciting coil has the referencenumber 2 and the sensor coil has the reference number 3, in each case byplacing the corresponding components in parentheses.

The coil 1 (or the exciting coil 2) is first used as a magnetizing coiland is excited by means of a magnetizing current for generating amagnetic field. If the bolt 4 is made of a ferromagnetic material, themagnetization field has the effect that the elementary magnets arealigned by the magnetic field of the coil 1 (or the exciting coil 2)through which the current flows. As known, the reversal of theelementary magnets is an erratic process (see FIG. 2) which results in achange of the magnetic flux, whereby a tension change is induced in thecoil 1 (or in the sensing coil 3). The start of the reversal oralignment of the elementary magnets is a function of the magnetizingfield intensity (or of the magnetizing current) and internal mechanicalstress in the material. (A tensile stress applied to the bolt 4 opposesthe alignment of the elementary magnets, or causes the elementarymagnets to start to reverse once again, into their original alignment.)This reversal of the individual elementary magnets, in turn, generates achange of the magnetic flux, whereby currents (the so-called Barkhausennoise) are induced in the coil 1 (or in the sensing coil 3). Since themagnetic field intensity (or the magnetizing current) required tomagnetize the bolt 4 is also a function of the internal mechanicalstress, the stress/strain condition of the bolt 4 can be determined. Inother words, a continuously rising magnetizing current generates acontinuously rising magnetic field in the coil 1 (or the exciting coil2), and the magnetizing current present at the start of the Barkhausennoise is a measurement of the tensile stress applied to the bolt 4.

As a rule, a pulsed magnetizing current is used to keep the thermalloading of the coil 1 (or of the exciting coil 2) as low as possible.Simultaneously, the start of the Barkhausen noise is monitored as afunction of the magnetizing current through the coil 1 (or through thesensor coil 3). The measurement takes place inductively, the coil 1 ineach case being set during the off-time of the pulses of the magnetizingcurrent to receive the Barkhausen noise. For a precise determination ofthe stress/strain condition, comparative measurements, determinedbeforehand bolts (or rivets, etc.) made of the same material and of thesame geometry, are used as a reference. The stress/strain condition istherefore determined by comparative measurements with previouslydetermined measured values, for example, electronically filed in atable.

In addition, it should be noted that, during the measurement of thestress/strain condition of, for example, a ferromagnetic bolt, a washermade of a non-magnetic or non-magnetizable material should be usedwhich, in the following, analogous to FIG. 1, has the reference number5′.

The method according to the invention, can also be used to determine thestress/strain condition of a non-magnetic or non-magnetizable bolt 4′.In this case, a washer 5 made of a magnetic or magnetizable material isused, so that the stress transmitted from the bolt 4′ to the washer 5 iscorrespondingly determined in the manner described above.

As an alternative, instead of inductive determination by means of asensing coil 3, the Barkhausen noise can be detected by means of anacoustic or interferometric detector 7, illustrated schematically inFIG. 3. The principle on which the invention is based is unchanged, onlythe Barkhausen noise is determined in a different fashion. The detector7 may, for example, be a microphone or a piezoelement, to mention only afew examples.

FIG. 4 shows another alternative measuring arrangement on an example ofa riveted connection. In FIG. 4, a rivet 8 is connected with thestructure 6; and an exciting coil 2 and the sensing coil 3 areillustrated separately as wire-wound coils around a core 9. The excitingcoil 2 is again acted upon by a variable magnetization, and the sensingcoil 3 detects induced stress by flipping over the domains. Instead ofthe sensing coil 3, an acoustic or interferometric detector 7 can alsobe used here for the detection of the Barkhausen noise.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

1. A method of measuring stress/strain of magnetic or magnetizableelements by detecting Barkhausen noise, comprising: arranging anexciting/sensing device at least adjacent to said element; applying acontinuously rising magnetizing current to the exciting device;detecting starting of Barkhausen noise by means of the sensing device;determining magnitude of the magnetizing current when the Barkhausennoise starts; comparing the determined magnitude of the magnetizingcurrent when the Barkhausen noise starts with measured reference valuesto determine the stress/strain condition of the element; and outputtinga signal representing the determined stress/strain condition of theelement.
 2. The method according to claim 1, wherein theexciting/sensing device is arranged in a manner in which it at leastpartially surrounds the magnetic or magnetizable element.
 3. The methodaccording claim 1, wherein an intermediate element made of anon-magnetized or non-magnetizable material is arranged between themagnetic or magnetizable element and a structure that is to be connectedtherewith.
 4. The method according to claim 1, wherein: saidcontinuously rising magnetizing current is pulsed; and the sensingdevice detects the Barkhausen noise during off-time of the pulses. 5.The method according to claim 3, wherein the magnetic or magnetizableelement is arranged between a non-magnetic or non-magnetizable fasteningelement and a structure that is to be connected therewith, before thedetermination of its stress/strain condition.
 6. The method according toclaim 5, wherein the determined magnetizing current at the start of theBarkhausen noise is proportional to the internal stress of the element.7. A method of measuring stress/strain in a magnetic or magnetizableitem, comprising: applying a continuously increasing magnetic field tosaid item; detecting a time of commencement of Barkhausen noisegenerated in said item in response to said magnetic field; determiningstrength of said magnetic field at said time of commencement ofBarkhausen noise; determining stress/strain in said item as a functionof the determined strength of the magnetic field at said time ofcommencement of Barkhausen noise; and outputting a signal representingthe determined stress/strain condition of the element.
 8. The methodaccording to claim 7, wherein said applying step comprises passing acontinuously increasing magnetizing current through an exciting devicesituated in proximity to said item.
 9. The method according to claim 8,wherein said step of determining strength of the magnetic fieldcomprises measuring said magnetizing current at said time ofcommencement of Barkhausen noise.
 10. The method according to claim 9,wherein said step of determining stress/strain in said item comprisescomparing measured magnetizing current with measured reference valuesfor said item.